Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/93

 fallacious aspect of opacity in consequence of innumerable minute fractures by which they are pervaded throughout; they are most commonly white, but the flesh coloured variety is also found: their magnitude varies from the twentieth to that of three tenths of an inch in breadth.

Stilbite is perhaps the most abundant of these substances which Sky produces, and it appears to be the most generally diffused throughout the island; it occurs along the shore which I have described, but in less quantity than in the northern district. It is so common in some places in the northern of Kilmuir and Snizort that it is scarcely an exaggeration to say, that the roads are sometimes almost made of it. In some situations the decomposed trap falling into a powdery soil, leaves large accumulations of it resisting the action of weather long after the rock has mouldered away, while in other places it has been converted itself into a friable mass, which, as I already remarked, has been mistaken for marle and used as manure.

It presents scarcely any varieties of crystallization: the predominant, I might almost add the universal form, is that most common one consisting of very flat tetrahedral prisms, terminating in tetrahedral pyramids, of which the faces are placed on the edges of the prism. These are aggregated in distinct fasciculi, parallel or divergent, of which the groups sometimes affect the form of the constituent crystals. In the neighbourhood of Loch Eynort I observed some specimens of great beauty, consisting of large and distinct square prisms terminated at each extremity by truncated, tetrahedral pyramids arising from the edges, the crystals being transparent and nearly an inch in length, adhering slightly by its side to the quartz crystals of the chalcedonic nodule in which it was formed.